Enzymes are used for certain industrial applications. Enzymes are catalysts that speed up the rate of reaction between substances without themselves being consumed in the reaction occur except at temperatures so high they would threaten the well-being of the body. Enzymes are proteins. Changes in the pH or acidity of the environment can alter or completely inhibit an enzyme from catalyzing a reaction. This change in the pH can affect the polar and non-polar intramolecular attractive and repulsive forces and alter the shape of the enzyme and the active site such that the substrate molecule can no longer fit. Correcting pH or temperature imbalances will usually allow the enzyme to resume its original shape or conformation. Some substances, however, will break enzymatic bonds and disrupt its primary structure so that the enzyme is inhibited permanently. Many toxic substances break covalent bonds and cause the unraveling of the protein enzyme. Other toxic substances precipitate enzymes. Urea is commonly used to denature proteins.